Doherty amplifiers are commonly used to amplify signals in systems that require the efficient conversion of direct current (DC) power to modulated radio frequency (RF) power. For example, in cellular and other RF applications, base stations or other infrastructure components employ Doherty amplifiers to broadcast signals over great distances.
A typical Doherty amplifier topology includes multiple amplifier stages that operate in parallel to supply current to a load (e.g., an antenna). For example, a two-stage Doherty amplifier includes a main amplifier stage and a peaking amplifier stage. At input power levels below the threshold of the peaking amplifier stage, only the main amplifier stage provides current to the load. At input power levels exceeding the threshold of the peaking amplifier stage, currents output from both the main and peaking amplifier stages are summed in-phase to provide current to the load. More specifically, the peaking amplifier stage is biased to turn on when the input signal increases above a level that would cause the main amplifier stage to saturate. An output impedance network coupled to the outputs of the main and peaking amplifier stages is configured so that the apparent impedance seen by the main amplifier stage decreases when the peaking amplifier stage is producing current. This enables the main amplifier stage to deliver more current in conjunction with the current delivered by the peaking amplifier stage.
To ensure that the currents from the main and peaking amplifiers are summed in-phase, some Doherty amplifiers also include an input impedance network configured to apply a phase shift to the input signal supplied to the peaking amplifier stage. In a particular topology, at the input to the Doherty amplifier, the input signal is split into two channels, and a phase shift (typically a quarter wave) is applied to the signal carried on the channel corresponding to the peaking amplifier stage. The output impedance network aligns the phases of the output signals produced by the main and peaking amplifier stages by applying a similar phase shift to the output of the main amplifier stage prior to summing the outputs of the main and peaking amplifier stages.
To achieve desired performance of a Doherty amplifier, the main and peaking amplifier stages are designed asymmetrically. More particularly, the transistor associated with the peaking amplifier stage typically is larger (e.g., twice as large) as the transistor associated with the main amplifier stage. Accordingly, producing a Doherty amplifier design typically involves twice the design effort than is required to design an amplifier with a single transistor or with matched transistors. In addition, many current Doherty amplifier designs suffer from relatively poor DC-to-RF conversion efficiency and signal quality, and/or relatively large design footprints.